This invention relates to an ink-jet printing method and apparatus for performing printing by jetting ink onto a recording medium from a print head.
A recording apparatus such as a printer, copier or facsimile machine is adapted to form dots on a recording medium such as paper or thin plastic sheets by individual recording elements (nozzles, heating elements or wires, etc.), thereby printing an image comprising the dots. Such a recording apparatus can be classified, depending on the recording technique used, as an apparatus of ink-jet type, wire dot type, thermal type or laser beam type, etc. Among these, the apparatus of the ink-jet type (referred to as an ink-jet printer) is so adapted as to jet ink (the recording fluid) from the nozzles of a print head and cause the jetted ink to adhere to a recording medium to thereby form an image on the medium.
A number of studies have been made for the purpose of improving the tonality of a color graphics output when printing a color image using such an ink-jet printer. For example, an improvement proposed and put into practice in recent years involves either raising print performance by making printing resolution higher than that of an ordinary color printing mode or raising the resolution of the printing apparatus, sending multilevel image data to the printing apparatus as print data and providing a multilevel printout using dots of a size different from the ordinary dot size. (The dots of the different size are referred to as xe2x80x9csubpixelsxe2x80x9d.)
An example of a method using subpixels is one which prints an image using mixed dots, namely dots of large and small size. Such a printing method makes it possible to improve tone reproducibility in image formation. However, though this method can be implemented with ease if the number of nozzles possessed by the ink-jet head is one per color, control becomes very complex if use is made of a head having a plurality of nozzles for each color.
In order for each nozzle to jet ink, the nozzle ordinarily is driven at a frequency above several kilohertz. Though direct control by a CPU is possible if the number of nozzles possessed by a head is small, it becomes increasingly necessary in view of processing speed to make joint use of hardware circuitry such as a gate array as the number of nozzles is increased. Further, if the amount of ink jetted from the nozzles is to be modulated to form large and small dots, this is carried out by changing the width of the driving pulses for ink discharge or by changing over the timing at which the driving elements within the nozzle are driven to discharge the ink. If drive timing is changed over, it is required that the head be internally provided with two drive-data registers per nozzle, one register being for large dots and one for small dots. If the number of nozzles is increased, the number of registers also increases by a whole-number multiple. The end result is print head circuitry of large scale and an attendant increase in the cost of the print head.
The method of changing the driving pulse width requires the provision of individual signal lines in order to control the nozzles individually. Consequently, the single signal line that usually suffices becomes several hundred signal lines (which is equivalent to the number of nozzles). This makes necessary an equivalent number of contacts between the head and its cable, an equivalent number of lines in the flexible cable leading to the print head and an equivalent number of driver transistors for the recording elements. This in turn leads to much higher cost.
If one foregoes printing by mixing large and small dots by way of single scan of a serial print head, then printing is carried out by causing the print head to make a plurality of scans (multiple passes) and combining scans which form large dots and scans which form small dots. Such method makes it possible to print by mixing large and small dots in an image through a simple arrangement. However, since this method always requires plural scans of the print head, a longer period of time is needed for printing.
Further, in a situation where pixels are thus formed using large and small dots, it is desirable to form the pixels by causing the large and small dots to overlap. However, a problem which arises is that the large and small dots are formed at positions offset from one another. This causes the image to take on a grainy appearance owing to small dots formed in spaced-apart relation and results in the appearance of white stripes.
Accordingly, an object of the present invention is to provide an ink-jet printing method and apparatus through which a pixel can be printed by a plurality of dots conforming to the tone of the pixel using a simple arrangement and simple control.
Another object of the present invention is to provide an ink-jet printing method and apparatus for modulating the amount of jetted ink in order to jet ink for forming dots of different diameters, and printing an image by a plurality of dots whose diameters conform to the levels of multilevel print data.
A further object of the present invention is to provide an ink-jet printing method and apparatus capable of printing dots conforming to the tones of printed pixels without reducing printing speed.
Yet another object of the present invention is to provide an ink-jet printing method and apparatus through which it is possible to form an image in which the occurrence of graininess and white stripes is suppressed by forming pixels using overlapping dots of large and small diameters to form the pixels.
According to the present invention, the foregoing objects are attained by providing an ink-jet printing apparatus in which ink jetted from an ink nozzle of a print head is made to adhere to a recording medium to form a pixel on the recording medium by the adhered ink, comprising:
scanning means for scanning the print head, which has a plurality of the ink nozzles, in a main-scan direction; and drive means, provided in correspondence with each nozzle of the print head, capable of successively jetting at least two inks of mutually different velocities from the ink nozzles at prescribed timings in synchronization with scanning of the print head by the scanning means in order to form the pixel from a plurality of dots; wherein distance between the ink nozzles and the recording medium, the prescribed timings at which the at least two inks are jetted and the velocities at which the at least two inks are jetted are controlled so as to satisfy a predetermined relationship in order that the at least two inks successively jetted from the print head by the drive means at the prescribed timings will adhere to the recording medium within the pixel.
Further, the foregoing objects are attained by providing an ink-jet printing method in which ink jetted from an ink nozzle of a print head is made to adhere to a recording medium to form a pixel on the recording medium by the adhered ink, comprising: a scanning step of scanning the print head in a main-scan direction; and a drive step of successively jetting at least two inks of mutually different velocities from the ink nozzles of the print head at prescribed timings in synchronization with scanning of the print head in order to form a plurality of dots which form the pixel; wherein distance between the ink nozzles and the recording medium, the prescribed timings at which the at least two inks are jetted and the velocities at which the at least two inks are jetted are controlled so as to satisfy a predetermined relationship in order that the at least two inks successively jetted from the print head by the drive step at the prescribed timings will adhere to the recording medium within the pixel.